1. Field of the Invention
The invention relates to a current balancing circuit, more particularly to a current balancing circuit for a discharge lamp unit.
2. Description of the Related Art
In recent years, discharge lamps such as cold cathode florescent lamps, external electrode cold cathode fluorescent lamps, etc., have been widely used in liquid crystal display devices for providing source light. When a plurality of discharge lamps are coupled in a parallel configuration, it is relatively difficult to ensure balance among the currents flowing therethrough due to impedance differences among the discharge lamps, thereby resulting in unbalanced luminance of the source light.
As shown in FIG. 1, as disclosed in European patent publication no. EP1,517,591(A1), a first conventional current balancing circuit for a discharge lamp unit 11 includes a step-up transformer 13 and a current balancer 14. The discharge lamp unit 11 includes first, second, third, fourth, fifth and sixth discharge lamps 111˜116. One end of each of the first, second, third, fourth, fifth and sixth discharge lamps 111˜116 is grounded.
The step-up transformer 13 is adapted to be coupled electrically to a power supply 12 for receiving an alternating-current source power therefrom, and for generating a drive signal by varying magnitude of the alternating-current source power.
The current balancer 14 includes first, second, third, fourth and fifth shunt transformers 141˜145, each of which includes primary and secondary windings. One end of each of the primary and secondary windings of the first shunt transformer 141 is coupled electrically to the step-up transformer 13 for receiving the drive signal therefrom. The other end of the primary winding of the first shunt transformer 141 is coupled electrically to one end of each of the primary and secondary windings of the second shunt transformer 142. The other end of the primary winding of the second shunt transformer 142 is coupled electrically to one end of each of the primary and secondary windings of the third shunt transformer 143. The other ends of the primary and secondary windings of the third shunt transformer 143 are adapted to be coupled electrically to the other ends of the first and second discharge lamps 111, 112, respectively. The other end of the secondary winding of the second shunt transformer 142 is coupled electrically to one end of each of the primary and secondary windings of the fourth shunt transformer 144. The other ends of the primary and secondary windings of the fourth shunt transformer 144 are adapted to be coupled electrically to the other ends of the third and fourth discharge lamps 113, 114, respectively. The other end of the secondary winding of the first shunt transformer 141 is coupled electrically to one end of each of the primary and secondary windings of the fifth shunt transformer 145. The other ends of the primary and secondary windings of the fifth shunt transformer 145 are adapted to be coupled electrically to the other ends of the fifth and sixth discharge lamps 115, 116, respectively.
Numbers of turns of the primary and secondary windings of the first shunt transformer 141 have a ratio of 1:2, such that the currents flowing through the primary and secondary windings of the first shunt transformer 141 have a ratio of approximately 2:1. The primary and secondary windings of each of the second, third, fourth and fifth shunt transformers 142˜145 correspond to each other in number of turns thereof, i.e., the numbers of turns of the primary and secondary windings of each of the second, third, fourth and fifth shunt transformers 142˜145 have a ratio of 1:1. Therefore, the currents flowing through the primary and secondary windings of each of the second, third, fourth and fifth shunt transformers 142˜145 have a ratio of approximately 1:1. Consequently, differences among the currents flowing through the first to sixth discharge lamps 111˜116 are small.
Although (P) parallel-connected discharge lamps can be driven with balanced currents by providing a current balancer with shunt transformers that are connected in a tournament tree structure as described above, and by suitably setting the ratio of the numbers of turns between the primary and secondary windings of each of the shunt transformers, it should be noted that (P−1) shunt transformers are required for such a configuration.
As shown in FIG. 2, as disclosed in U.S. Patent Application Publication No. 20050093472, a second conventional current balancing circuit for a discharge lamp unit 21 includes a step-up transformer 23 and a current balancer 24.
The discharge lamp unit 21 includes first, second, third, fourth, fifth and sixth discharge lamps 211˜216. One end of each of the first to sixth discharge lamps 211˜216 is grounded.
The step-up transformer 23 is adapted to be coupled to a power supply 22 for receiving an alternating-current source power therefrom. Since the step-up transformer 23 has functions similar to those in the first conventional current balancing circuit (shown in FIG. 1), further details of the same are omitted herein for the sake of brevity.
The current balancer 24 includes first, second, third, fourth, fifth and sixth shunt transformers 241˜246, each of which includes primary and secondary windings. One end of the primary winding of each of the first to sixth shunt transformers 241˜246 is coupled electrically to the step-up transformer 23 for receiving the drive signal therefrom. The other ends of the primary windings of the first to sixth shunt transformers 241˜246 are adapted to be coupled electrically to the other ends of the first to sixth discharge lamps 211˜216, respectively. The secondary windings of the first to sixth shunt transformers 241˜246 are coupled electrically to each other in a serial ring configuration.
The first to sixth shunt transformers 241˜246 have identical ratios of number of turns between the primary and secondary windings thereof. Since the secondary windings of the first to sixth shunt transformers 241˜246 are coupled electrically to each other in the serial ring configuration, currents flowing therethrough are nearly identical, such that the currents flowing through the primary windings of the first to sixth shunt transformers 241˜246 are nearly identical. Consequently, differences among the currents flowing through the first to sixth discharge lamps 211˜216 are small.
However, for (P) parallel-connected discharge lamps to be driven by the second conventional current balancing circuit, (P) shunt transformers are needed.
As shown in FIG. 3, as disclosed in U.S. Pat. No. 6,781,325, a third conventional current balancing circuit for a discharge lamp unit 31 includes a step-up transformer 33, and a current balancer 34. The discharge lamp unit 31 includes first, second, third, fourth, fifth and sixth discharge lamps 311˜316. One end of each of the first to sixth discharge lamps 311˜316 is grounded.
The third conventional current balancing circuit differs from the second conventional current balancing circuit in the current balancer 34. The current balancer 34 includes first, second, third, fourth and fifth shunt transformers 341˜345, each of which includes primary and secondary windings. One end of the primary winding of the first shunt transformer 341, and one end of the is secondary windings of each of the first to fifth shunt transformers 341˜345 are coupled electrically to the step-up transformer 33. The other end of the primary winding of the first shunt transformer 341 is adapted to be coupled electrically to the other end of the first discharge lamp 311. The primary winding of the second shunt transformer 342 is adapted to be coupled electrically between the other end of the secondary winding of the first shunt transformer 341 and the other end of the second discharge lamp 312. The primary winding of the third shunt transformer 343 is adapted to be coupled electrically between the other end of the secondary winding of the second shunt transformer 342 and the other end of the third discharge lamp 313. The primary winding of the fourth shunt transformer 344 is adapted to be coupled electrically between the other end of the secondary winding of the third shunt transformer 343 and the other end of the fourth discharge lamp 314. The primary winding of the fifth shunt transformer 345 is adapted to be coupled electrically between the other end of the secondary winding of the fourth shunt transformer 344 and the other end of the fifth discharge lamp 315. The other end of the secondary winding of the fifth shunt transformer 345 is adapted to be coupled electrically to the other end of the sixth discharge lamp 316.
The primary and secondary windings of each of the first to fifth shunt transformers 311˜315 correspond to each other in number of turns thereof. Therefore, the currents flowing through the primary and secondary windings of each of the first to fifth shunt transformers 311˜315 have a ratio of approximately 1:1. Consequently, differences among the currents flowing through the first to sixth discharge lamps 311˜316 are small.
However, for (P) parallel-connected discharge lamps to be driven by the third conventional current balancing circuit, (P−1) shunt transformers are required.
In other words, although each of the first to third conventional current balancing circuits is capable of providing balanced currents to a plurality of discharge lamps that are connected in parallel, manufacturing costs are kept high due to the large number of shunt transformers required therein.